Synonym Physomonas Kent (ref. ID; 1618)
[ref. ID; 1618]
Similar to Ochromonas, but without chromatophores; active and plastic; often attached to foreign objects; body small, up to 20 um long; fresh and salt water. (ref. ID; 1618)
[ref. ID; 6733]
Muller (1773, 1786) was the first who applied the Linnean system to microorganisms. Still, the smaller forms (including the nanoflagellates) were pooled by Muller in the newly erected genus Monas (Vermis inconspicuus, simplicissimus, pellucidus, punctiormis [inconspicuous, most simple, transparent, and punctiform worm]). The genus Monas sensu Muller (1773) roughly corresponds to the family Monadaires of Bory (1824). Still, Bory defined the Monadiares exclusively negatively as having no caudiform appendages. He alreadly differentiated four genera within the Monadaires with genus Monas comprising spherical forms. Bory shifted the oblong forms to the genus Lamellila and other forms such as Monas ocellus to the genus Ophtalmoplanis. Ehrenberg, in his pioneering work of 1838, further split this group in the Monadina and Cryptomonadina (Ehrenberg 1838) families, defining the family Monadina as motile organisms without visible appendages and external organs at a magnification of at least 300x. The genus Monas sensu Ehrenberg was much narrowly defined as the original genus of Muller, but it was still a complication of organisms of small size and insufficient morphological features. Ehrenberg's genus Monas comprised all Monadina, which lack a tail ("Schwanz"), a lip ("Lippe"), an eye ("Auge") and do not build clusters and do not aggregate (one or two flagellate [misidentified as "Russel"(trunk) by Ehrenberg] may be present). As the differential to Ehrenberg's aggregated genera Uvella and Polytoma is not strict, several species most probably belonging to Monas were also placed in the former genera. In 1841, Dujardin not only allowed for the presence of flagella in Monas and in the family Monadiens, but also explicity defined this genus and the family by the possession of flagella (Dujardin 1841). With that, he changed the original negative criterion (no appendages) to a positive criterion (flagella present in the Monadiens, and specifically one single anterior flagellum in the genus Monas). The increased taxonomic resolution had already led to the segregation of some of the most prominent of the 'ubiquitous' nanoflagellate taxa, specifically of the genera Bodo Ehrenberg (1832) and Cercomonas Dujardin (1841). Stein restricted the family Monadina to unpigmented forms (Stein 1878) and considered that all reddish Monas species belonged to fungi while brownish Monadines of Ehrenberg belonged to the newly established family Chrysomonadina. Despite the increasingly narrower definitions of the nanoflagelle taxa (and specifically of the Monadina and the genus Monas), these taxa were still criticized as being a collection of indifferently intermingled flagellate forms. Kent (1880-81) provided a more detailed subdivision of these simple nanoflagellates. He differentiated the organisms according to the number of flagella with the Monadinae (within the Pantostomata-Monomastiga) having, by definition, only one flagellum. Therefore, many organisms formerly affiliated with the Monadinae or even the genus Monas were excluded, specifically the genera Physomonas Kent 1880-1881, Heteromita Dujardin 1841, Polytoma Ehrenberg 1832, Goniomonas Stein 1878 (all described as having two flagella) and the genera Spumella Cienkowsky, 1870 and Tricomonas Donne, 1836 (described as having three flagella).
Electron microscopy further provided morphological characteristics allowing for a clear separation of some taxa formerly linked to the genus Monas (such as Paraphysomonas spp. cf. Preisig and Hibberd 1982, 1983). More and more taxa came into question or were shifted to other genera. Whereas Reynolds (1934) still accepted 13 species of Monas, only three of the more than 100 binomina affiliated with the genus Monas were considered to be valid by Silva (1960). Silva (1960), and later Preisig et al. (1991), proposed the generic name Spumella instead of Monas, as the type species of the latter genus is questionable. But even the genus Spumella is problematic, for two reasons: first, only very few strains of Spumella were subject to electron microscopic investigation (Belcher & Swale 1976; Bruchmuller 1998; Preisig and Hibberd 1983; Mignot 1977; Tanichev 1993), and second, it remained unclear whether the strains investigated were identical or even related to the original type species of Spumella (on Monas). The strains of Spumella investigated so far were all characterized by a naked cell surface; heterokont flagella emerging from an apical depression with mastigonemes on the long flagellum, while the short flagellum in naked; and mitochondria with tubular cristae. A number of other structures differed between strains; for instance, they may or may not possess a leucoplast, a flagellar swelling, an eyespot or mucocysts (Bruchmuller 1998; Preisig and Hibberd 1983). However, as a result of the breakdown of the taxon Mastigophorea, affiliaton with the genus Monas became even less clear, as different taxa formerly affiliated with the genus Monas were now related to different major taxonomic groups. Largely due to the observation of siliceous cysts in Spumella (and in related Monas spp. e.g. Scherffel 1924) and the increasing synonymous use of both generic names (Preisig et al. 1991; Reynolds 1934; Silva 1960), a consensus view emerged placing the genus Monas within the chrysophytes/chrysomonads despite the lack of taxonomic evidence. In summary, electron microscopic studies confirmed the great diversity of protists and were thus a major step towards a separation and characterization of major eukaryotic lineages; the phylogenetic relationships between these lineages and their evolution remained, however, ambiguous (Karpov 2000). Similarly, on the species and genus level, electron microscopy contributed to the designation of (new) taxa. However, regarding the genus Monas/Spumella (and a number of other flagellate taxa) electron microscopy neither proved the monophyly of the respective genus nor clarified the identity of the types species. Even though electron microscopy may have had the potential to sufficiently separate paraphyletic lineages within these aggregate genera, the sparseness of ultrastructural studies and the predominance of light microscopic analyses in environmental studies did not provide a sufficient database for sound conclusions about phylogenetic relationships. (ref. ID; 6733)